The "S" model is the more serious off roader. It has longer travel suspension, hence more ground clearance. Don't know how tall you are Nordic but even the basic model's a fairly tall bike... There may be some other differences, gearing etc, but I'm not sure. I don't think you'd need the "S" for the kind of riding you're thinking of in all honesty.
Sounds like ideal territory for the Adventure. All the reviews I've seen reckon it's surprisingly good on the road, where most of them spend most of their time, but also really capable off road, especially on the kind of roads you're talking about.
When I had an Adventure as a courtesy bike whilst the SuperDuke got serviced I liked it a lot. It has a ton of road presence, you don't see many, it's really well put together and a hoot to ride.
The 950 version, the first one, was carbed and smooth as anything. The 990 is fuel injected and suffers somewhat from sudden throttle response from a closed throttle, just as the SD can. My advice would be to ride the 990 and see if it'll be a problem, especially as you're thinking of taking it off road and therefore using smaller throttle openings. The good news is that KTM dealers in the UK at least are very good about test rides
Bet you don't want to ride the Harley after you try the Katoom
If you want more off road potential there's also a 950 Enduro, if you want more luncay potential there's the 950 Super Motard.
And if you want accessories KTM do everything from extra lights through dash mounted road books to KTM flip flops. Oh yes!
So many bikes, so little time
Mitch